Refactor IDE to allow RightPane extension

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21920

What does this MR do?

• Refactors the IDE to allow extending the RightPane component with additional tabs.
• Also causes the RightPane to use v-if initially to prevent rendering content, and then use v-show after a tab has been clicked for the first time. (This will be handled in another MR)

What are the relevant issue numbers?

#5426 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?